The definition of visual impairment is conventionally based on the visual acuity and vision. Visual impairment means that the visual acuity after correction is below 0.3 (6/20). The legally blindness means that, after correction, the visual acuity is still lower than 20/200 or the visual field is less than 20 degree. Usually, even when one is classified as blind, he still may have slight capacity of distinguishing light and dark. Low vision is used to describe those who don't have complete vision however have better visual acuity than total blind. According experts, “low vision” is defined as having quite a visual impairment however having quite a little of useful residual vision. The way one uses visual aids to enhance the effective use of his residual vision and the condition one's visual impairment are very positively relevant. Such as the age of onset, the visual impairment occurs at different ages will result in different capacity of learning, thinking pattern concerning concrete and abstract objects, skills, concepts etc. Some diseases also cause visual lesions. For example, the loss of central vision caused by macular degeneration will affect one's reading ability while the loss of peripheral vision caused by retinitis pigmentosa or glaucoma will affect one's moving ability.
The main function of current low-vision aid system, such as closed-circuit television (CCTV), magnifying glass, telescope, the magnification software for computers, etc. is to enlarge fonts or images, is generally used in static environment However, once the fonts or images are magnified, the visual field is narrowed. As to color images, there is no enhancement in luminance contrast. The current low-vision aiding tools are thus only suitable for static indoor use and cannot help low-vision people to increase indoor or outdoor moving ability.
Hence, how to enhance the visually distinguishing ability for low-vision people is one of the main tasks that the industry tries to accomplish.